This invention relates to weighing scales utilizing strain gage load cells and more particularly to an improved method and apparatus for compensating such a weighing scale for changes in load position.
It is well known that inaccuracies may arise in weighing when the objects to be weighed are placed at different positions on a weight receiving platter. Such inaccuracies are referred to as "load position errors" and may be compensated in various ways. One of these methods has been to desensitize appropriate ones of the load cell strain gages connected in a bridge circuit. This is accomplished by connecting resistors of appropriate value in circuit with certain of the strain gages.
In U.S. Pat. No. 3,576,128 to Lockery, a dual beam load cell is compensated for changes in load position along the longitudinal axis of the beams by connecting shunt resistors across the strain gages at one end or the other of the load cell beams. In U.S. Pat. No. 4,380,175 to Griffen, a dual beam load cell is compensated for errors caused by changes in load position both parallel to the longitudinal axes of the beams and transverse to the axes by displacing the strain gages on the beams and connecting resistors of appropriate value in circuit with the strain gages. Such techniques have provided substantial improvement in compensating for load shift errors. A disadvantage in the use of resistors, however, remains in that resistors are typically manufactured in fixed values which rarely correspond to the values required for load shift compensation. This can result in the increased labor and expense of utilizing a number of resistors in parallel to arrive at the correct value. Further, the techniques for determining the correct compensating resistance values may be complicated and time consuming.
Recently, a weighing scale has been developed which, because of its design, requires load shift compensation of only one of its two load cells and only for load position changes parallel to one axis of the beams. This weighing scale is described and claimed in U.S. patent application Ser. No. 994,938, filed Dec. 22, 1992, and assigned to the Assignee of the present invention. The present invention is of particular use in scales such as this which require load shift compensation in only one direction.